1. Field of the Invention
This is a further development in thermionic detection techniques, and provides a method and apparatus for flameless thermionic detection of specific substances that thermally decompose into electronegative species.
2. Prior Art
In 1936, J. P. Blewett (Physical Review, Vol. 50, p. 464, 1936) described studies in which efficient filament sources of positive ions of the alkali metals were produced by heating synthetic alkali aluminum silicates. These alkali-glass sources were shown to be abundant emitters of positive ion current and poor emitters of negative ion current, especially when heated to temperatures close to the melting temperature of the alkali-glass.
In 1951, Rice (U.S. Pat. No. 2,550,498) described a method and apparatus for electrically detecting vapors of certain substances by sensitizing a hot surface with a material from the class of alkali metals and their compounds, bringing sample vapor into contact with the hot surface, and measuring the current of positive ions produced at the hot surface by the presence of the sample vapor. A preferred embodiment of Rice's apparatus consisted of two concentric platinum cylinders with appropriate diameters such that gas vapors of interest could flow through a gap between the inner and outer cylinders. The cylinders were electrically biased so as to cause the motion of positive ion current in the direction from the inner cylinder to the outer cylinder. The inner cylinder further surrounded a helical heater coil wound on an alumina cylinder. Natural alkali impurities within the alumina ceramic served to produce the required sensitizing action for a relatively short operating time. Rice taught that the active life of a sensitized alumina could be increased or restored by soaking the alumina in a water solution containing an alkali metal salt. For even longer life, Rice further taught that the alumina cylinder could be replaced by a cylinder of alkali glass such as one of those described by Blewett. The detector described by Rice was found to be especially effective in the detection of compounds containing halogen atoms.
In 1957, Roberts (U.S. Pat. No. 2,795,716) described an improved electrical vapor detector which used a positive ion source that provided a relatively long life compared to that source described in the Rice patent. The positive ion source described by Roberts consisted of a cylindrical alumina ceramic core upon which was wound a heater coil. The alumina core and heater coil were covered on their outer surfaces with a coating of positive ion emitting material. For the positive ion emitting material, Roberts used the alkali-glasses described by Blewett. The alkali-glass was powdered and mixed with a suitable ceramic cement in the desired proportion, and the mixture was coated on the alumina core and heater coil and allowed to set.
In 1975, Kolb and Bischoff (U.S. Pat. No. 3,852,037) described a selective ionization detector which used an electrically-heated alkali-galss bead maintained in a heated softened state during operation of the detector. Kolb and Bischoff theorized that the softened glass state acted to maintain an adequate supply of alkali to the surface of the glass by means of molecular motion within the body of the glass. Kolb and Bischoff described a detector in which the alkali glass bead was mounted above a burner nozzle to which a mixture of a combustible gas and a sample gas was supplied. A collecting electrode was located above the glass bead, with an electrical bias applied between bead and collector so as to direct negative ion current to move from the bead to the collector. Kolb and Bischoff further taught that specificity for particular individual substances could be obtained by suitable selection of gas flows and by selection of the appropriate alkali metal used in the alkali-glass bead. For example, rubidium-glass appeared best suited for detection of nitrogen compounds, whereas sodium-glass was especially good for phosphorus compounds.
In 1977, Burgett et al. (Journal of Chromatography, Vol. 134, p. 57, 1977), described a new nitrogen-phosphorus detector for gas chromatography. The active component in this detector was described as a ceramic cylinder coated with an alkali salt activator similar to that described by Rice. Actually, this alkali-ceramic cylinder has the physical appearance of a ceramic core covered by a glass-like outer shell, similar to the positive ion source described by Roberts. Like Rice and Roberts, the alkali-ceramic cylinder in Burgett's detector was suspended in the center of a collector cylinder and positive ion current arriving at the collector was measured. Electrically, one end of Burgett's alkali-cylinder was connected to the collector cylinder, and the other end was connected to a source of electrical heating power. The electrical potential difference between the alkali cylinder and the collector electrode was mostly provided by an electrical fringe field caused by biasing the collector at a high voltage with respect to a flame jet structure external to the collector.
The nature of the ionization mechanism operating in these prior art devices has not been well established. However, both Rice and Kolb and Bischoff theorized that the probable ionization mechanism involved release of neutral alkali atoms from the alkali source and subsequent gas phase ionization of the alkali vapors by reaction with sample compounds. Consequently, according to prior art teachings, the alkali source was considered as serving mainly to supply neutral alkali vapors to the gaseous environment of the source.
Those devices of the prior art which sensed positive ion current suffered from the fact that when heated, the alkali-sensitized sources were themselves abundant positive ion emitters even without the presence of a sample. Consequently, there always existed in these positive ion detectors a large background signal which acted to mask the responses obtained from samples. This high background level also was highly susceptible to noise variations caused by changes in such things as gas flows or contamination.
The devices described by Rice, Roberts, and Burgett et al. involved sensitized elements in which only the surface layers contained the activating alkali compound. Consequently, the lifetimes of these sensitized elements were limited by the depletion of active material from the surface layers.
In the alkali-glasses described by Blewett and Kolb and Bischoff, the alkali metal was present throughout the body of the glass; and Kolb and Bischoff theorized that active material in the glass surface layer was continually replenished by migration of alkali atoms from within the body of the glass. However, such synthetic alkali-glasses are difficult to manufacture because a glass melt must be made starting from dry ingredients. Furthermore, if a particular shape of the alkali-glass bead is desired, that shape must be formed while working with the glass in the molten state. A special complication in manufacturing, forming, and operating alkali-glass beads is the fact that the softening and melting points of the glass depend strongly on the type and density of the alkali metal used in the glass formulation. This property severely limits the freedom of manufacturing alkali-glass beads of widely varying formulations. In general, increasing the alkali atom content of a glass usually results in a decrease in melting point. Consequently, alkali-glass beads are often restricted in use at high temperatures due to the onset of glass melting. In fact, Kolb and Bischoff have taught that the alkali-glass beads in their device must be operated above the glass softening point for satisfactory operation. In the Kolb and Bischoff device, only a slight overheating of the bead is often sufficient to cause physical destruction of the bead by melting.
In the devices described by Kolb and Bischoff, and by Burgett et al., the electric field established between the alkali-bead and the collector electrode is highly non-uniform, and may appropriately be described as an electrical fringe field. As a consequence, the response characteristics of these two prior art devices are known to be highly dependent on the precise spatial location of the alkali-bead with respect to the collector or any other electrode that is at a different voltage from the bead.